Oracle does not support partial indexes but since Oracle does not index NULL values you can emulate it with a functional index. (Only really useful if used for unique indexes, otherwise to kludgey.) I do not think DB2 supports partial indexes either, and I have no idea if they can be emulated.
The only two I know support them are PostgreSQL and MSSQL.
Oracle has index sharding though - would that help in the use case of a very large table where typically only a specific, small chunk of it is accessed?
The only two I know support them are PostgreSQL and MSSQL.